1. Field of the Invention
The present invention relates to an automotive automatic transmission and, more particularly, to a cut-back pressure control device for a multi-speed automatic transmission which is comprised of a main transmission unit and a sub-transmission unit.
2. Description of the Related Art
Generally, an automatic transmission has a torque converter and a planetary speed changing gear mechanism. The planetary speed changing gear mechanism is comprised mainly of an overdrive (O/D) planetary gear unit, a front planetary gear unit and a rear planetary gear unit. The planetary speed changing gear mechanism performs, under the control of two solenoid valves and three shift valves, a speed change over four forward speeds and one reverse speed.
The known automatic transmission of the type explained above has a cut-back valve which is adapted to apply a cut-back pressure to the throttle valve. The hydraulic pressure which is supplied to a 2nd brake hydraulic servomotor B.sub.2 at second or a higher speed is delivered also to the cut-back valve, so that the cut-back valve interrupts the cut-back pressure when at first speed or in reverse and delivers the cut-back pressure when second or a higher speed is selected.
Japanese Laid-Open Patent No. 37140/1982 of the present applicant proposes a transmission control device incorporating three solenoid valves and three shift valves, adapted for controlling the operation of a six-speed automatic transmission which consist of a combination of a sub-transmission unit comprised of an overdrive planetary gear unit and a main transmission unit comprised of a front planetary gear unit and a rear planetary gear unit.
In this automatic transmission, six speeds are obtained by suitably combining two speeds provided by the sub-transmission unit, namely, overdrive speed and direct-coupling speed, and three speeds provided by the main transmission unit, namely, 1st, 2nd and 3rd speeds. It is to be pointed out that the six-speed automatic transmission mentioned above encounters the following problem when the pressure to the 2nd brake B.sub.2 is used as the signal pressure for operating the cut-back valve as in the known automatic transmission. Namely, when the automatic transmission as a whole is upshifted to the second speed by an upshift of the sub-transmission from the direct-coupling speed to the overdrive speed while the main transmission unit selects the first speed, the cut-back pressure is not supplied so that the throttle pressure and, hence, the line pressure are set at an unnecessarily high level, resulting in an inferior shock tuning during the upshifting and a loss of power due to unnecessary load on the hydraulic pump.